From the viewpoint of environmental problems and resource conservation, there is a demand for a reduction in energy loss due to a friction in a sliding member such as a valve system component and a piston constituting an engine. For this reason, various surface treatments have been performed on a sliding surface of the sliding member in order to reduce the frictional coefficient of the sliding member and to improve the wear resistance thereof. Among these, an amorphous hard carbon film called a diamond-like carbon (DLC) film is widely used as a film that improves the slidability of the sliding surface.
In order to improve the original characteristics of the DLC film and adopt other characteristics, for example, the DLC film is added with a metal component or is formed to be layered with a film having other characteristics. As the cited documents for the layering of the DLC film, Patent Documents 1 to 5 may be exemplified. Patent Documents 1 to 5 all disclose a coating film having low friction and excellent wear resistance. All the coating films have a layered structure. For example, the layered structure is formed by layering a DLC film having different characteristics, layering a DLC film added with other components, or layering a DLC film and another compound film.
For example, Patent Document 1 discloses a coating layer obtained by repeatedly and alternately layering a hard carbon film added with at least one kind of metal component along with at least one kind of metal, metal carbide, metal nitride, or metal carbonitride. Specifically, a layered film may be obtained by alternately layering a hard carbon film added with boron and a hard carbon film added with silicon, a layered film may be obtained by alternately layering titanium carbonitride and a hard carbon film, and a layered film may be obtained by alternately layering a hard carbon film added with tungsten and a hard carbon film added with boron. In Patent Document 1, since a sliding action is supposed under the non-lubricant environment in which lubricating oil is not used, it is considered that lubricating oil is not used in a wear test for a sample including such a layered film. The interaction between the component contained in the layered film and the component contained in the lubricating oil largely influences the frictional wear characteristics, but in Patent Document 1, such an interaction is not considered at all.
In addition, Patent Documents 2, 3, and 5 also disclose a layered film, but even in such cited documents, an evaluation in lubricating oil was not performed. Meanwhile, in Patent Document 4, a sliding test is performed by using lubricant containing molybdenum dialkyldithiocarbamate (Mo-DTC). However, the film included in the sliding member disclosed in Patent Document 4 is a DLC multi-layered film which contains carbon and hydrogen, and essentially does not contain other components. For this reason, the interaction between the component added to the film and the component contained in the lubricating oil is not considered.
Further, in order to reduce the frictional coefficient of the existing sliding member including the DLC film containing the metal component, a use in lubricating oil containing molybdenum added with Mo-DTC and the like is assumed in many cases. However, in the sliding action in the lubricating oil containing molybdenum, the oxidational wear of the DLC film occurs, and hence the wear is promoted compared to the sliding action in lubricating oil that does not contain molybdenum or the sliding action in an environment without lubricant. Further, there is a concern that the use of the lubricating oil containing the heavy-metal molybdenum may cause environmental problems. For this reason, there is a demand for a sliding member that exhibits a low frictional coefficient in lubricating oil without molybdenum.